484 Rev. J. F. Blake — Precamhrian Geology. 



I am not aware that there are any courses (in beds parallel to the 

 rest) of black shale ; the faults which let down the masses which 

 are said to have yielded Graptolites may be seen on both sides. If 

 there are thinner black shale bands which I have not observed, that 

 is, no doubt, a point to be considered ; but it could only be decisive 

 if they contained Graptolites of known Silurian species. 



The Director sums up by saying that this area "is proved by the 

 evidence of fossils at its base, towards its centre, and at its top to 

 belong to the Lower Silurian series," This sounds like a settled 

 matter. But may I ask (or rather can I get an answer to the 

 questions ?) What is the lowest bed of the series above the Black 

 Shales ? What are the names of the supposed Silurian fossils it 

 contains ? and where may they be seen ? All Sir A. Ramsay says 

 is, '' On the north by the shore a few poor fossils in irregular bands 

 of limestone clearly indicate the Caradoc or Bala age of some of 

 the beds." What are the names of the Graptolites which occur 

 towards the centre ? and what is the proof that Or this Bayleana is 

 a Bala fossil ? 



The arguments generally for the Monian age of these northern 

 rocks are these : The volcanic group is more or less repeated in 

 various parts of the island. It must be admitted, however, that those 

 that most resemble these northern rocks do not clearly show their 

 stratigraphical relations. The quartz knobs and limestone masses 

 which are found amongst them also characterize parts which are 

 admittedly Precambrian. It may be said against this that these 

 may be of several ages ; but it is strange that they occur only in 

 Anglesey and amongst these rocks. Bala rocks are known in the 

 island as black shales proved to be Bala by their named fossils (see 

 Mem. Geol. Survey, vol. iii. p. 225), including Orthides ; but none of 

 these are 0. Bayleana, and it would be strange that , the volcanic 

 rocks of different ages should be so similar north and south, and 

 rocks of the same age so different in the centre. If all these 

 arguments could be met and the volcanic group were accepted as 

 Bala, the next alternative would be that of Sir A. Earasay, to 

 consider part Silurian and part Cambrian, the line of separation 

 being obliterated by the squeezing. Otherwise, if all were Bala, 

 we should have identical chloritic schists at Llanflewin and Abersant, 

 the former Bala, the latter overlaid unconformably by Arenig grit, 

 to say nothing of similar rock overlaid by the same at Carmel's Point 

 and Treiorwertb. 



I have next to notice the reasons given for rejecting the term 

 "Monian" for a series of rocks in Scotland, of which it can be 

 written, " No one familiar with the Dalradian rocks of Scotland and 

 Ireland can fail to be struck with the close resemblance which these 

 younger Anglesey schists bear to them, down even to the minutest 

 details," and "if we are justified in grouping these Anglesey rocks 

 with the Dalradian schists." I will not say that, if by " Dalradian " 

 is meant the whole of the seventeen groups enumerated, these reasons 

 as a whole 'are not well founded, since in that case it may include 

 more than one system, of which, to judge from the statements made 



